1. Field of the Invention
This invention relates to a gas generator that utilizes the combustion of a solid gas generant composition for the rapid generation of an extremely clean gas in which most of the combustion particle residue and noxious and offensive gases have been removed. The generator is particularly useful for rapidly filling vehicle inflatable cushion restraint systems for the protection of the occupants from severe impact and possible injury during a collision.
2. Description of the Prior Art
Gas generators that utilize combustible solid gas generant compositions are known in the prior art. Common features of such generators that are used for the inflation of cushion restraint systems are the inclusion in a housing of a gas generant composition containing a pelletized alkali metal azide and means to filter and to cool the gas positioned between the composition and gas discharge orifices, as defined by the housing. Such compositions are considered advantageous for use in such applications because the product of combustion is mainly nitrogen, an inert gas, and the speed of reaction or burning thereof, upon ignition, may be selected to be such as to effect the generation of gas at a very rapid rate but without detonation. Such constituents that burn rapidly, however, usually burn with a temperature of about 1000.degree. C., and develop a pressure in the housing in excess of 2000 psia. Further, the gas that is generated contains combustion residue including molten particles of metal and/or reactant oxides and also noxious and offensive gases. The high pressure that builds up in the housing tends to produce an explosive discharge of residue from the housing.
Accordingly, among the problems associated with such prior art gas generators has been the containment of the high pressure reaction within a housing of acceptable weight and bulk, and adequate filtering and cooling of the generated gas. Various chemical and mechanical cooling means that have been provided have been found to be either unreliable after extended periods, unacceptably bulky, and/or incapable of reducing the generated gas temperature to a desirably low level. The filtering means that have been provided have also been incapable of reducing the combustion residue entrained in the gas to a desirably low level. In each case the result is discomfort to the vehicle occupants who have been spared severe impact during a collision.
A proposal made in the prior art for reducing the temperature of the generated gas, as disclosed in U.S. Pat. No. 3,985,076, granted Oct. 12, 1976, is to promote both the cooling and speed of the reaction by a mechanical arrangement of a pelletized gas generant composition, ignition means therefor, and means for filtering and the cooling, thereby allowing the use of a cooler burning gas generant. That is to say, the gas generant composition is disposed to surround the ignition means so that a greater proportion of the gas generant material is directly exposed to the hot gases and flame therefrom. Cooling and the speed of reaction are enhanced by the fact that the gases generated expand rapidly in all directions rather than linearly, as in a cylindrical arrangement. While an improvement in these respects, there still exists a need for further improvements in respect of providing a gas that is generated at even lower temperatures and lower pressures, also, and with less entrained combustible residue and noxious and offensive gases.
Gas generating compositions have been proposed that comprise mixtures of metal azides, oxidant metal compounds and silicon dioxide. The stated purpose of the silicon dioxide is to react with and thereby transform the toxic solid combustion residue to a non-toxic or physiologically harmless residue, specifically a glass-like alkali silicate. Known U.S. patents that disclose such compositions are U.S. Pat. No. 3,883,373, granted May 13, 1975, No. 3,912,561, granted Oct. 14, 1975, No. 3,947,300, granted Mar. 30, 1976, and No. 4,021,275, granted May 3, 1977. Such compositions leave something to be desired because of difficulty in uniformly mixing silicon dioxide with the metal azide and oxidant compound as required to react the silicon dioxide with the residues and for obtaining satisfactory combustibility.
Mechanical filtering systems are also known that include filter packs in the gas-flow path, such filter packs including multiple layers of screen of various mesh sizes, some made of metal and including one or more layers of high temperature glass fibers in which the filtering action is determined solely by the mesh sizes of the various filter layers. U.S. patents disclosing such filter packs are U.S. Pat. No. 4,017,100, granted Apr. 12, 1977, and No. 4,012,211 granted Mar. 15, 1977.
There is also described in the prior art gas generant compositions that are said to burn with the production of only the desired gaseous product and a solid product in the form of a sinter or clinker that does not escape from the generator housing. Such compositions are described in U.S. Pat. No. 3,895,098, granted July 15, 1975, No. 3,931,040, granted Jan. 6, 1976, No. 3,996,079, granted Dec. 7, 1976 and No. 4,062,708, granted December 13, 1977. They are described as comprising mixtures of metal oxides such as nickel oxide or iron axide, and an alkali metal azide. A primary particle size for the reactant oxide in the range of a small fraction of a micron to a few microns is indicated as essential for effecting a burning rate fast enough for inflating an inflatable occupant restraint system.
Efforts to make an operative gas generator utilizing a gas generant composition as described in the aforementioned patents have been unsuccessful. Among the problems encountered were difficulty in compacting the mixture to form a stable pellet, and difficulty in igniting the mixture.
Pelletizing the gas generating composition has been found to be essential for the composition to remain reliable over extended periods, and for providing a uniform surface area for uniform burning upon ignition of the composition. Otherwise the burning rate is not predictable. Without pelletizing there is a tendency for packing and separation of the finely divided particles after the gas generator has been subjected to vibration over an extended period of time, as occurs during ordinary use, particularly when applied to a vehicle.
In copending application of Fred E. Schneiter and George F. Kirchoff, Jr. bearing Ser. No. 970,602, filed Dec. 18, 1978, and issued on May 20, 1980 as U.S. Pat. No. 4,203,787, there is described a gas generant composition consisting of a mixture of 60-80% of sodium azide (NaN.sub.3) 2-26% or iron oxide (Fe.sub.2 O.sub.3), 0-6% of sulfur (S.sub.2) and 2-26% of molybdenum disulfide (MoS.sub.2), a preferred mixture comprising 66% sodium azide, 30% iron oxide, 2% sulfur and 2% molybdenum disulfide. It has been found that this mixture is readily compacted to form a stable pellet and provides a burning rate at a combustion temperature of 1025.degree. C., that is fast enough to effect inflation of a vehicle cushion restraint system within 45 to 60 milliseconds. Important features of the combustible composition are its substantially lower operative burning temperature at a high burning rate and at a substantially lower pressure, providing a gas, as generated, that is cooler by at least 200.degree. C. than that available from prior art generators. The sodium azide produces a high percentage of nitrogen generating efficiency. The molybdenum disulfide and sulfur provide excellent combustion stability. The iron oxide is reduced during combustion, and the iron, Fe, that is thereby produced, forms a shower of molten particulate. This particulate tends to be blown out of the combustion chamber, being carried along with the generated gas. There remains, additionally, combustible residue in the form of offensive gases that also tend to be carried out of the generator with the generated gas.
Thus, there still exists a need for further improvements in solid fuel gas generators, particularly in respect of the filter structure and its cooperative relation with the gas generant composition and the combustion chamber, at the high temperature environment therein, for enabling the trapping within the combustion chamber of larger amounts of the particulate matter and for rendering innocuous the noxious and offensive gases that are produced, thereby to make the gas generator more attractive to both the user and the manufacturer, to the user from the standpoint of comfort as well as safety, and to the manufacturer from the standpoint of simplifying and reducing the weight and size of the structure, and hence, reducing the manufacturing cost, and further, enhancing reliability.